TPU and bubbles in the printing

[Danielrug05 0]

Hi, a few weeks ago i printed some parts in TPU or flex and today i saw the parts like this:

As you can see on the surface some bubbles started to appear. I stored the parts in a cardboard box for a week or so and i dont know if that could cause this inconvenient. Although, is there any cleaning substance that can cause this?

Thanks

[kmanstudios 1,120]

That looks like pillowing. Maybe not enough top and bottom layers as it allowed the ambient pressure to build up and push on the thin layers over the air pockets?

Yes, I am taking a wild guess here. Just a shot in the dark. But it does look like pillowing.